Generation of the cells that constitute the blood elements, including red blood cells, granulocytes and macrophages, platelets, and lymphocytes is crucial for both metabolic and immune functions. Genetic and acquired defects in hematopoiesis are responsible for multiple problems including leukemias, anemias, bleeding abnormalities and immune dysfunctions such as AIDS. Additionally, the lymphohematopoietic system is often damaged, sometimes irreversibly, by pharmacologic and other therapeutic agents, especially those used for the treatment of cancer. The lymphohematopoietic cells are constantly replenished by a common precursor, called the hematopoietic stem cell. Despite a large body data on the biology of these cells, there is a paucity of information regarding the molecular mechanisms that regulated stem cell functions. We postulate that hematopoietic stem cell proliferation and differentiation is under genetic control. Two approaches will be taken to identify such regulatory genes. Preliminary evidence demonstrates that AKR/J mice have three-six fold more hematopoietic stem cells than C57B1/6 mice. In anticipation of a positional cloning efforts, a genetic screen will be initiated to map the location of these genes. As a second approach, this laboratory has developed methods to generate libraries of large cDNAs from small populations of cells. This will allow us to construct cDNA libraries from pure populations of hematopoietic stem cells with self renewal capacity from both BA and AKR/J mice, and from pluripotent hematopoietic progenitor cells without self renewal capacity. These libraries will be analyzed by the laboratory of Dr. L. Hood, and genes specifically expressed by each population will be identified. The functions of these genes will be determined by the laboratory of Dr. I. Weissman. The following specific aims are designed to accomplish these goals. First, to determine the genetics of the inheritance of the increased hematopoietic stem cell number in AKR/J mice. Second, to characterize genes expressed by the hematopoietic stem cells of AKR/J and Ba (C57B1/6) mice. Third, and finally, to extensively characterize the genomic region(s) surrounding the hematopoietic stem cell frequency gene(s) such that a positional cloning effort can be initiated. Identification of these genes will lead to a better understanding of the factors that modulate hematopoietic stem cell replication and differentiation in vivo.